Drive for drafting arrangement rolls of long spinning machines

ABSTRACT

A drive for the rolls defining a main drafting zone of drafting arrangements of long spinning machines, wherein in order to prevent drafting defects caused by distortion movement of the rolls, the latter are driven from one end by a first gear arrangement and from the other end are interconnected rigidly drivewise or slippage-free, respectively, during the standstill of the spinning machine via a second gear arrangement and via a freewheel clutch.

BACKGROUND OF THE INVENTION

The present invention concerns a drive for the driven draftingarrangement rolls of long spinning machines extending over the fulllength or over considerable parts of the length of the machine,preferably of ring spinning machines, in which the drafting arrangementrolls arranged on the same longitudinal side of the machine and defininga main drafting zone for the fibre slivers to be drafted, are mutuallyconnected at one of their ends by first slippage-free transmissionelements which effect the ratio of rotational speeds of these rollsdetermining the draft ratio.

With textile machines of this type there is a tendency, active since thevery origins, to increase the number of working positions, e.g. of thespindles in the case of the ring spinning machine, driven by one singledrive headstock. This tendency is prompted mainly by economic factors,as the increase in the number of working positions per machine resultsin a reduction of the price per working position, in a reduction ofspace per working position and in most cases, also in operatingadvantages.

One difficulty in achieving the above mentioned increase, however, ispresented by the bottom rolls of the drafting arrangements of suchmachines, as in excessively long rolls the deformations caused bytorsion can result in unacceptable working conditions. This concernspredominantly the rolls defining the main drafting zone of the draftingarrangement, i.e. e.g. the middle roll and the delivery roll of thedouble apron drafting arrangement, as widely used today on variousspinning machines, preferentially on ring spinning machines, but alsoe.g. on the roving frame. In a main drafting zone of such type, thefibre mass is drafted to its final fineness before twist is imparted ata draft ratio of 10 fold and more, normally of about 30 fold. Anydistortion, however small, of one roll active in the main drafting zoneof a drafting arrangement with respect to the other roll immediatelycauses a considerable drafting defect resulting in a thick place and ina thin place in the drafted yarn. In many cases then very undesirableyarn breakages result. Experience with especially long spinning machineshas shown, that the danger of distortion of the roll of the draftingarrangement is particularly great for the middle roll of a double aprondrafting arrangement. Onto this roll, which as a rule also acts as adeflecting roll for the bottom apron of the double apron draftingarrangement, particularly strong braking forces act tangentially, whichconsiderably exceed similar forces acting on the delivery rolls.

This difference is due to the rotational speed, which is reduced withrespect to the delivery roll by the draft ratio, and due to the higherfriction forces mainly generated by the apron which acts on this roll,and to the strong drafting forces acting on this roll in the predraftingzone. Due to these higher braking forces the middle drafting roll duringthe operation of the drafting arrangement thus is distorted to a higherdegree than the delivery roll. If now the spinning machine is stopped,the forces acting tangentially upon the roll, and particularly theimportant drafting forces, are released to a large extent, in suchmanner that the input roll of the main draft zone, which is moredistorted with respect to the delivery roll of the main drafting zone inthe backward sense, tends to reverse this distortion and distort back inthe normal sense of rotation. As the machine is thereafter started up,drafting defects generated in this manner already cause yarn breakages.

The situation described here for the drafting arrangement of theconventional spinning machine is further exacerbated in that during thesubsequent start-up of the machine higher static friction forces are tobe overcome by the input roll of the main drafting zone than by thedelivery roll, which forces cause progressive distortion from thebeginning of the input roll to its end. Also due to this distortion, thedegree of which exceeds the distortion during normal operation, draftingdefects and thus yarn breakages arise.

A reduction of the distortion mentioned by increasing the diameter ofthe rolls is excluded because of spinning technology requirements as anincrease in diameter could be achieved only to the detriment of thequality of the fibre control during the drafting process.

Thus, it has been proposed already to avoid the above mentioneddisadvantage of a long spinning machine by either driving the draftingrolls separately at both ends, in which arrangement the drafting rollscan be divided all the way or at the middle of the machine, as e.g.shown in U.S. Pat. No. 3,339,361 or by interconnecting the rolls atleast in pairs via separate toothed gear drives, as shown in GermanDE-OS No. 26 41 434. All these proposed arrangements are suitable foravoiding the undesirable distortion of the rolls; they show, however,substantial disadvantages. Thus, they require a complete and exactlysynchronous double drive for the drafting rolls. This solution isexpensive, and still more important, it implies, at least in anarrangement using continuous rolls, a very dangerous source of errors.If the draft ratio in the main drafting zone, which is to be adapted bycorrespondingly choosing gears (draft change gears) or similar elements,due to operator's error is not set to the same value at both side,torsion-forced breakage of one of the rolls inevitably occurs. Thissolution thus requires highest attention of the operating personnel andthus runs contrary to the intentions of the spinning mills offacilitating easy and error-free operation of the spinning machine.

SUMMARY OF THE INVENTION

It thus is an object of the present invention to eliminate the mentioneddisadvantages of the known solutions, and in particular to propose adrive for the drafting arrangement rolls of the above mentioned type, inwhich the continuous drafting arrangement rolls of the main draftingzone cannot mutually distort backward during the standstill of themachine, without requiring the rolls to be driven at both ends, or to bemutually interconnected.

This object is achieved, using a drive of the above mentioned type, byproviding at the other end of the drafting arrangement rolls second,slippage-free transmission elements comprising a freewheel clutch, theratio of rotational speeds of which elements is lower than the ratio ofrotational speeds determining the draft ratio, which during normaloperation of the machine permit either running behind or lag of thefaster running roll or a lead of the slower running roll of the draftingarrangement, and which during the standstill of the faster running rollof the drafting arrangement effect a slippage-free connection via thetransmission elements.

This solution achieves the result that, upon reaching machine standstilland upon release of load in the drafting arrangement, the determinedtorsion of the drafting arrangement roll built up during the operationof the machine, and in particular the input roll of the main draftingzone, which is strongly distorted in the lagging sense cannot distortback, but that the distortion is maintained also after the forcesgenerating it (friction in the bearings, drafting forces) are released.This is achieved since during the standstill of the machine the draftingarrangement rolls at both ends are mutually interconnected slippage-freedrive wise, namely on one end by the actual drafting arrangement geardrive, i.e. by the first slippage-free transmission elements, and on theother end by the second slippage-free transmission elements which, viathe now activated freewheel clutch, are rigidly coupled drivewise. It isparticularly advantageous that coupling of the rolls always is effectedautomatically at the exact moment at which the rolls stop, independentlyof the ratio of rotational speed prevailing between the rolls,determined by the first slippage-free transmission elements, i.e.independently of the drafting ratio. From this advantage results, thatif the drafting ratio in the main drafting zone is changed, theoperating personnel are not required to take care of the secondslippage-free transmission elements at the other end of the rolls, avery dangerous source of errors thus being eliminated.

It has proven to be advantageous to choose the ratio of rotationalspeeds of the second slippage-free transmission elements just slightlylower than the ratio of rotational speeds of the first slippage-freetransmission elements determining the minimum draft ratio chosen for thespinning machine. This measure ensures that the difference in rotationalspeeds to be overcome in the idling direction is kept as small aspossible, in such manner that also the wear of the freewheel clutch iskept to a minimum. Furthermore, the proposed solution also provesadvantageous with respect to the small play always present inslippage-free transmission elements (e.g. the play of the gear toothflanks, if such transmission elements are provided), as defects causedby the backward distortion due to said play are kept to a minimum.

According to an alternative design example of the inventive drive theslower running roll of the drafting arrangement, i.e. the input roll ofthe main drafting zone, can be connected at its end provided with thesecond slippage-free transmission elements, with an element distortingthe roll in the sense of its normal rotation using a second freewheelclutch, in such manner that the second freewheel clutch does not impairthe rotation of the drafting arrangement roll during its normaloperation, as it is overhauled, whereas during the standstill of themachine it effects the slippage-free connection.

Owing to this arrangement it is possible to eliminate the higherdistortion of the slower running roll caused during the start-up of themachine by the higher static friction compared to the sliding or dynamicfriction. Due to the higher static friction or due to the higher"breaking loose resistance" of the roll respectively, the slower runningroll tends to lag behind with respect to the faster running roll, causedby a distortion in addition to the one suffered during the normaloperation of the machine, i.e. it is rotated only after a certain timelag. This again causes a drafting defect, which depending on thedistortion, is higher the longer the distance from the drive of therolls is, e.g. the longer the rolls are. Even if this additionaldistortion of the slower running rolls is levelled out again immediatelyafter the "breaking loose" of the rolls, it still can cause yarnbreakages. It is possible for the slower running roll of the draftingarrangement at its end, at which the second transmission elements arearranged, to be connected via a second freewheel clutch with an elementforcing the roll in its normal direction of rotation, and that thesecond freewheel clutch during the normal operation of the machine doesnot impair the rotation of the roll of the drafting arrangement whereasduring the standstill of the machine it effects the slippage-freeconnection. By virtue of these it can be achieved, that the slowerrunning drafting roll during the start-up of the machine is alwayspulled, so to speak towards the faster running roll at the stop, in suchmanner that it "breaks loose" always simultaneously with the fasterrunning roll. The drive arrangement with the second freewheel clutchthus has the function of eliminating the effect of the differencebetween static friction acting on the slower running roll and the fasterrunning roll and of ensuring that both rolls are started alwayssynchronously. This measure, however, is only required, if the abovementioned difference in the effect of static friction is practicallyeffective in the inventive drive, which depends to a large extent on thebearing type used.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive drive is explained in the following in more detail withreference to illustrated design examples. There is shown in

FIG. 1: a schematic, partially perspective view of a ring spinningmachine with the inventive drive of the drafting arrangement rolls,

FIG. 2: a simplified, perspective view of an alternative design exampleof the drive, and

FIG. 3: the drive according to the alternative design example shown inFIG. 2, seen in the direction of arrow A of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 the elements of a spinning position of a ring spinning machineare shown schematically. They consist, as seen in the direction ofmaterial flow, of a roving bobbin 1 rotatably suspended from a rigidcreel support 2, which supplies the roving 3 to be processed to adrafting arrangement. The drafting arrangement comprises three bottomrolls 4, 5 and 6 with corresponding pressure top rolls 7, 8 and 9.Arranged subsequently are a thread guide 10, a combination of ring 11with a traveller 12 and a rotating spindle 13. The spindle 13 isrotatably supported in a ring rail 14 and supports a yarn bobbin 15,onto which the freshly spun yarn is wound, and is set into rotation by abelt 16.

The strand of fibres emerging from the pair of delivery rolls 6, 9 ofthe drafting arrangement, drafted to the required fineness is twisted inknown manner under formation of a balloon 17 into a yarn and is woundonto the yarn bobbin 15.

The drafting arrangement comprises a first drafting zone (pre-draftzone) limited by the pairs of rolls 4, 7 and 5, 8 and a second draftingzone (main drafting zone), defined by the pairs of rolls 5, 8 and 6, 9.In the first drafting zone the roving 3 is drafted only slightly, e.g. 1to 2 fold only, i.e. the difference in surface speed of the slowerrunning rolls 4 and 7 with respect to the one of the faster runningrolls 5 and 8 is small. Between the pairs of rolls 5, 8 and 6, 9 thefibre roving is drafted at a high draft ratio to the final yarnfineness. This second drafting zone thus is called the main draftingzone and the draft effected therein as a rule ranges between about 10fold and 40 fold, in special cases up to 100 fold and more.

For better control of the fibre mass during the drafting operation inthe main drafting zone as a rule so called double apron arrangements areused, here consisting of a bottom apron 18 surrounding the bottom roll 5and of top apron 19 surrounding the top roll 8.

The bottom apron 18 as well as the top apron 19 are guided along themain drafting zone by suitable means (not shown) in FIG. 1 and aretensioned e.g. by tension rolls 20.

The fibre roving 3 now is guided between the legs or runs of the aprons18 and 19 which are in mutual contact and are running parallel, in suchmanner that the fibre control required for high drafts is ensured. Suchpairs of aprons 18, 19 are rotated only if considerable friction forcesare overcome by the drive force of the driven roll 5 of the draftingarrangement. The roll 5 forms the input roll of the main drafting zone,whereas the roll 6 also is called delivery roll of the main draftingzone, or of the whole drafting arrangement, respectively. For simplifieddefinitions the rolls 5 and 6 in the following description are referredto as the slower running roll and as the faster running roll,respectively, of the main drafting zone.

In a ring spinning machine referred to as long machine, about 250spinning positions or more are lined up along one machine side. Therolls 4, 5 and 6 extending over all spinning positions of a machine sidethus are of a length of about 18 to 35 m. For spinning technologyreasons the diameter of the rolls 4, 5 and 6 is limited to a maximum ofabout 30 mm, and thus also their distortion resistance is relativelylow.

The rolls 4, 5 and 6 of the drafting arrangement are driven from theright hand side by slippage-free transmission elements. The fasterrunning roll 6 of the main drafting zone is directly driven by a motor21 (sense of rotation according to arrow m).

The ratio of the numbers of teeth of the gears 24 and 22, the effectivediameters of the rolls 5 and 6 being taken into account (and thethickness of the apron 18 on roll 5 also being taken into account)represents the drafting ratio in the main drafting zone. Of course,instead of gears also other slippage-free transmission elements, such ase.g. chains or toothed belts, can be applied. The important factor isjust that the slower running roll 5 and the faster running roll 6 of themain drafting zone are interconnected by slippage-free transmissionelements determining the drafting ratio.

The roll 4 of the drafting arrangement can be set into rotation e.g.from the slower running roll 5 of the main drafting zone also viaslippage-free transmission elements, e.g. tooth gears 24, 25 and 26, inwhich gear train, according to the function of the pre-draft, the ratioof the numbers of teeth of the gears 26 and 24 is chosen small, normallyin the range from 1 to 2. The rolls 4, 5 and 6 of the draftingarrangement are rotatably supported by a large number of bearings, notshown in FIG. 1, evenly distributed along the spinning machine.

For rotating the rolls 4, 5 and 6 the braking moments, generated byfriction and causing a torque load on the roll, must be overcome. Theslower running roll 5 is subject to a higher torque than the fasterrunning roll 6, as the friction forces (e.g. generated by the apronassemblies) acting on it are considerably greater. Thus Also thedistortion of the roll 5 is greater than that of roll 6, in such mannerthat as the torque moments acting onto the rolls disappear or arereduced (e.g. as the load on the pressing rolls 7, 8 and 9 is released),the slower running roll 5 tends to distort back in the direction of thearrow m more than the faster running roll 6. For preventing differingreturn-distortion of the rolls 5 and 6 during the standstill of thedrafting arrangement, the rolls 5 and 6 are interconnected drive-wise attheir free left hand side ends via further slippage-free transmissionelements in such manner, that this connection is established only duringthe standstill of the faster running roll 6, whereas it remains inactiveduring operation. This is achieved in that a gear 27 rigidly mounted onthe roll 6 is provided. The roll 5, on the other hand is provided with afreewheel clutch 28, shown in FIG. 1 as a ratchet arrangement for thesake of clearer understanding. A freewheel clutch of such typefurthermore consists of an outer housing 29, which is connected with ahub 30 supported freely rotatable in bearings (not shown) on the roll 5and which is provided with teeth 31 inside for a ratchet 32. The ratchet32 is pivotably supported on the roll 5 and engages with the insideteeth 31 of the freewheel clutch 28 in such manner, that the outerhousing 29 can rotate freely relative to the roll 5 clockwise (as seenfrom the left hand side). Counter-clockwise its freedom of rotation withrespect to the roll 5 is blocked as the ratchet 32 engages with theteeth 31 on the inside, i.e. the clutch is engaged if rotated in thisdirection. The outer housing 29 furthermore is provided with a toothgear 33 on its periphery, which via an intermediate gear 34 is engagedwith the gear 27. In this arrangement the ratio of the numbers of teethof the gears 33 and 27 always is lower than the one mentioned beforewhich determines the draft ratio, between the gears 24 and 22. If aspinning machine is laid out for a determined range of draft ratios, itproves advantageous to choose the second mentioned ratio somewhatsmaller than the one determining the lowest draft ratio for which themachine is laid out, in such manner that the arrangement can function atany draft ratio set at a given time. Adaption of the transmission ratioto the main draft ratio, however, can be envisaged for the secondslippage-free transmission elements, e.g. by exchanging the gear 27.

The freewheel clutch 28, shown here as a ratchet arrangement with aninside ratchet, is not limited to this design type, however. A freewheelclutch of this type, chosen in FIG. 1 merely for the sake of simplicity,shows the disadvantage that it does not engage properly if the number ofteeth is too small. For optimum functional reliability of the inventivedrive a freewheel clutch with a small lost motion is to be provided,i.e. one which can engage at practically any position immediately.

The drive according to FIG. 1 now functions as follows:

During operation of the machine the outer housing 29 of the freewheelclutch is rotated clockwise by the gears 27, 34, 33 faster than theslower running roll 5 of the drafting arrangement, i.e. the freewheelclutch is disengaged. Before the rolls 5 and 6 come to a standstill theyare still distorted, the distortion of the slower running roll 5 beinggreater than that of the faster running roll 6. Now after stopping, e.g.by releasing the load on the pressure rolls 8 and 9, the friction forcesacting onto the rolls 5 and 6 are diminished. Thus the rolls 5 and 6tend to reduce their distortion, i.e. the roll 5 tends to distort backover a limited amount more than the roll 6 clockwise. This movement,however, of the roll 5 is blocked, as the ratchet 32 immediately engageswith the inside teeth 31 of the freewheel clutch 28. The outer housing29 thus also is driven clockwise for a limited rotation and tends totransmit this movement via the gears 33, 34, 27 to the stopped roll 6 ofthe drafting arrangement. The rolls 5 and 6 defining the main draftingzone thus are interconnected at both ends during the standstill of themachine via slippage-free transmission elements: and any relativerotation of the rolls, which would result in a drafting defect uponrestarting the machine, is excluded.

As soon as the spinning machine is started up again the blocking actionof the freewheel clutch 28 is released, as its outer housing 29 leadsthe slower running roll 5, i.e. rotates faster in the same directionthan the roll 5.

In FIGS. 2 and 3, in which the elements identical with the ones shown inFIG. 1 are designated with the same reference numbers, an alternativeembodiment of the inventive drive is shown, which differs from the oneshown in FIG. 1 only in that here the freewheel clutch 35, the functionof which corresponds to the clutch 28 of FIG. 1, is arranged on thefaster running roll 6 of the drafting arrangement. For simplifying thedrawing, only the two rolls 5 and 6 defining the main drafting zone ofthe drafting arrangement are shown. In FIG. 2 a perspective view of thedrive similar to the view illustrated in FIG. 1 is shown, whereas inFIG. 3 the drive according to FIG. 2 is shown as seen in the directionof arrow A.

In this alternative design example the freewheel clutch 35 is designedas a so called roller blocking arrangement with spring type frictionelements, i.e. a freewheel clutch which can engage practically withoutany lost motion.

A roller blocking arrangement of this type consists of an outer ring 36,which is rigidly connected with a concentrical gear 37 and is supportedfreely rotatable in bearings not shown on the roll 6 of the draftingarrangement.

The gear 37 meshes with an intermediate gear 38 which is engaged with agear 39 rigidly mounted on the slower running roll 5, and also in thisarrangement the ratio of the numbers of teeth of the gears 39 and 37 ischosen smaller than the corresponding ratio of the gears 24 and 22, i.e.is smaller than the main draft.

On the roll 6 a star wheel 40 is rigidly mounted and arranged within thehollow interior of the outer ring 36, which star wheel 40 is providedwith teeth 41 (FIG. 3) pivoted in the clockwise sense. Between the teeth41 rollers 42 are inserted which are pressed by springs 43 into thewedge-shaped chambers formed between the tooth intervals and the outerring 36. A roller blocking arrangement of such type, which iscommercially available, permits free rotation of the starwheel 40 withrespect to the outer ring 36 in one direction, in the example shown inclockwise direction, in the outer direction, however, any relativemovement is blocked as the rollers 42 jam, the jamming becomingpractically without any lost motion, as the rollers 42 are clampedimmediately.

The function of this drive corresponds substantially to that of thedrive described with reference to FIG. 1 and thus is not described inmore detail. In this arrangement the slower running roll 5 when it comesto standstill tends to distort back clockwise with respect to the fasterrunning roll 6 (always as seen from the lefthand side); this is blocked,however, by the action of the freewheel clutch 35, which immediatelyengages as its outer ring 36 is driven with respect to the star wheel 40clockwise.

During the normal operation of the spinning machine the outer ring 36,owing to the transmission described, is rotated slower than the starwheel 40, or the faster running roll 6, respectively, i.e. the freewheelclutch runs behind the roll 6.

The alternative design example according to FIGS. 2 and 3 furthermoreshows in which manner the slower running roll 5 of the draftingarrangement at its end, on which the second transmission elements 37through 39 are arranged, via a second freewheel clutch 44, which isidentical in its design with the freewheel clutch 35, is connected withan element 45, which distorts the roll 5 in the direction of its normalrotation, i.e. clockwise (as seen from the left hand side), in suchmanner that the freewheel clutch 44 during the normal operation of themachine does not impair the rotation of the roll 5 of the draftingarrangement, whereas it effects, as the machine comes to a standstill,the connection between the element 45 and the roll 5.

In the example shown in FIGS. 2 and 3 the element 45 consists of a lever46, which is rigidly connected to the outer ring 47 of the freewheelclutch 44 and a pneumatic cylinder 48, the piston rod 49 of which ispivotably connected with the other end of the lever 46.

The cylinder 48 is pivotably supported on an axle 50 which is fixedrelative to the room. The piston rod 49 is held during the operation ofthe spinning machine in its lefthand side position (shown with solidlines in FIG. 3) by a pressure spring, as the piston 51 is not subjectto pressure. In this position the outer ring 47 of the freewheel clutch44 thus remains at a standstill, whereas its star wheel 53 freelyrotates clockwise together with the roll 5.

The additional device described here merely serves for overcoming thestatic friction forces acting upon the slower running roll 5 of thedrafting arrangement during the startup phase of the spinning machine.

The additional device functions as follows: Before the spinning machineis started up the pneumatic cylinder 48 is pressurized. The piston 51with its piston rod 49 now tends to rotate the lever 46 clockwise. Asthe slower running roll 5 still is at standstill, the freewheel clutch44 is engaged. The lever 46 thus generates a torque moment in clockwisedirection (as seen from the left) acting onto the roll 5. The spinningmachine now is started up and the static friction forces acting upon therolls of the drafting arrangement are to be overcome. This is effectedby the torque moment mentioned which acts from the freewheel clutch 44onto the slower running roll 5 and transmitted via the gears 39, 38 and37 and the freewheel clutch 35, also acts onto the faster running roll6, which torque moment is to be considered as "breaking loose" moment.In this process the lever 46 is rotated over the angle α to the stop ofthe piston 51 (position of the lever 46 indicated with broken lines inFIG. 3). As the start-up of the spinning machine is completed, or as thelever 46 rests against its right hand side stop, the freewheel clutch 44is overtaken by its star wheel 53, in such manner that the rotationalconnection between the drive element 45 and the roll 5 of the draftingarrangement is released.

The pneumatic cylinder 48 in this arrangement can be activatedindependently in time from the spinning machine, i.e. it can beactivated at any time during the standstill of the machine. This,because the torque moment exerted onto the roll 5 of the draftingarrangement, owing to the inventive blocking of the roll 5 during thestandstill of the machine, cannot result in any distortion of the roll 5in this direction. The torque moment however, is always availableimmediately for the start-up of the machine. The dimensions of thecomponents of the element 45 (diameter and lift of the pneumaticcylinder 48, pressure in the cylinder, position of the lever 46, etc)depend on the static friction forces to be overcome and are chosenaccording to experiment.

While there are shown and described present preferred embodiments of theinvention, it is to be distictly understood that the invention is notlimited thereto, but may be otherwise variously embodied and practicedwithin the scope of the following claims. Accordingly,

What we claim is:
 1. A drive for the driven drafting rolls of a draftingarrangement of long spinning machines, wherein the drafting rolls extendover an appreciable part of the length of the machine, especially forring spinning machines, the driven drafting rolls of the draftingarrangement being arranged at the same longitudinal side of the spinningmachine, comprising:said driven drafting rolls defining a main draftingzone for a fiber sliver which is to be drafted and containing a fasterrunning roll and a slower running roll; said drafting rolls havingopposed ends; first substantially slippage-free transmission elementsfor mutually connecting said drafting rolls at their one opposed endsand for effectuating a ratio of rotational speeds of said drafting rollsin order to determine a predetermined draft ratio for said fiber sliver;second substantially slippage-free transmission elements provided forthe other opposed ends of the drafting rolls defining said main draftingzone; said second slippage-free transmission elements containing afreewheel clutch; said second slippage-free transmission elements havinga rotational speed ratio lower than the ratio of the rotational speedsdetermining said draft ratio; said second slippage-free transmissionelements, during normal operation of the spinning machine, selectivelyallowing either the faster running one of the drafting rolls of the maindrafting zone to lag or the slower running drafting roll of the maindrafting zone to lead; and said freewheel clutch during standstill ofthe faster running roll effecting a slippage-free connection between theslower running roll and the faster running roll via the said secondslippage-free transmission elements.
 2. The drive as defined in claim 1,wherein:said second substantially slippage-free transmission elementshave a ratio of their rotational speeds which is merely lower by a smallamount than the ratio of rotational speeds of the first slippage-freetransmission elements which determine a minimum draft ratio of thespinning machine.
 3. The drive as defined in claim 1, wherein:saidsecond slippage-free transmission elements comprise toothed gears whichmutually mesh with one another essentially free of play.
 4. The drive asdefined in claim 1, wherein:said slower running roll of the draftingarrangement has an end at which there is arranged the secondslippage-free transmission elements; means for forcing the slowerrunning roll into a predetermined normal direction of rotation. a secondfreewheel clutch for connecting said slower running roll at said endwith said forcing means; said second freewheel clutch during normaloperation of the spinning machine operating such as not to impair therotation of the slower running roll of the drafting arrangement whereasduring standstill of the spinning machine said second freewheel clutcheffects a slippage-free connection of said slower running roll with saidfaster running roll.
 5. The drive as defined in claim 4, wherein:saidforcing means comprise a lever; said lever having opposed ends; apneumatic piston-and-cylinder unit; one end of said lever beingoperatively connected with said second freewheel clutch; and the otherend of said lever being operatively connected with the piston of saidpneumatic piston-and-cylinder unit.